A flight instructor in the Irish Air Corps and the cadet he was training were two of many who have died as the result of an error in perception known as the somatogravic illusion.

While flying during bad weather, Captain Furniss thought his plane was climbing upwards away from a mountain when his plane was actually diving down into it.

The somatogravic illusion can occur when a pilot rapidly accelerates or decelerates. In Captain Furniss' case, it was when he was accelerating, in what's also called the "false climb illusion."

In our body, our inner ear tells our brain when our head is looking up. But this sensation of looking up can be mimicked when there is forward acceleration.

When a pilot feels this effect in his or her inner ear, the pilot's brain interprets this in one of two ways:

1) I am accelerating forward, or

2) I am moving at a constant velocity, but my head is tilted upwards.

As a pilot, if I believe my head is tilted upwards, then I will assume my aircraft is tilted upwards. Then I might try to correct this upward movement by directing my aircraft downward. Of course, what would really be happening is that I would be sending my aircraft downward towards the ground.

If I'm able to see something outside my aircraft, such as the horizon, I'll notice this illusion and correct my mistake. If I pay more attention to my instruments than the feeling in my body, I'll also be able to correct myself.

But if I don't take in any outside information - either because I don't watch or believe my aircraft's instruments, or because the sky outside is dark and I can't see the horizon - this illusion can be fatal.